U.S. patent number 7,986,090 [Application Number 11/908,632] was granted by the patent office on 2011-07-26 for light-emitting component.
This patent grant is currently assigned to Novaled AG. Invention is credited to Jan Birnstock, Sven Murano, Martin Pfeiffer, Gregor Schwartz.
United States Patent |
7,986,090 |
Pfeiffer , et al. |
July 26, 2011 |
Light-emitting component
Abstract
The invention relates to a light-emitting component, in
particular organic light-emitting diode, having an electrode and a
counterelectrode and an organic region--arranged between the
electrode and the counterelectrode--with a light-emitting organic
region, which comprises an emission layer and a further emission
layer and which, upon application of an electrical voltage to the
electrode and the counterelectrode, is formed in a manner emitting
light in a plurality of colour ranges in the visible spectral
range, optionally through to white light, in which case the
emission layer comprises a fluorescent emitter which emits light
predominantly in the blue or in the blue-green spectral range; the
further emission layer comprises one or a plurality of
phosphorescent emitters emitting light predominantly in the
non-blue spectral range; a triplet energy for an energy level of a
triplet state of the fluorescent emitter in the emission layer is
greater than a triplet energy for an energy level of a triplet
state of the phosphorescent emitter in the further emission layer;
and an at least 5% proportion of the light generated in the
light-emitting organic region is formed in the visible spectral
range as fluorescent light from singlet states of the fluorescent
emitter in the emission layer.
Inventors: |
Pfeiffer; Martin (Dresden,
DE), Schwartz; Gregor (Dresden, DE),
Murano; Sven (Dresden, DE), Birnstock; Jan
(Dresden, DE) |
Assignee: |
Novaled AG (Dresden,
DE)
|
Family
ID: |
34934266 |
Appl.
No.: |
11/908,632 |
Filed: |
February 22, 2006 |
PCT
Filed: |
February 22, 2006 |
PCT No.: |
PCT/DE2006/000328 |
371(c)(1),(2),(4) Date: |
December 03, 2007 |
PCT
Pub. No.: |
WO2006/097064 |
PCT
Pub. Date: |
September 21, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
|
US 20090230844 A1 |
Sep 17, 2009 |
|
Foreign Application Priority Data
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|
|
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Mar 15, 2005 [EP] |
|
|
05005573 |
|
Current U.S.
Class: |
313/504;
313/506 |
Current CPC
Class: |
H01L
51/5016 (20130101); H01L 51/5036 (20130101); H01L
51/002 (20130101); H01L 51/0051 (20130101); H01L
51/0062 (20130101); H01L 51/0059 (20130101) |
Current International
Class: |
H01J
1/62 (20060101); H01J 63/04 (20060101) |
Field of
Search: |
;313/504,506
;428/690 |
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|
Primary Examiner: Patel; Nimeshkumar D
Assistant Examiner: Zimmerman; Glenn
Attorney, Agent or Firm: Sutherland, Asbill & Brennan,
LLP
Claims
The invention claimed is:
1. A light-emitting component comprising an electrode and a
counterelectrode and an organic region--arranged between the
electrode and the counterelectrode--comprising a light-emitting
organic region, which comprises an emission layer and a further
emission layer and which, upon application of an electrical voltage
to the electrode and the counterelectrode, emits light in a
plurality of colour ranges in the visible spectral range, wherein:
the emission layer comprises a fluorescent emitter which emits
light predominantly in the blue or in the blue-green spectral
range; and the further emission layer comprises one or a plurality
of phosphorescent emitters emitting light predominantly in the
non-blue spectral range; a triplet energy for an energy level of a
triplet state of the fluorescent emitter in the emission layer is
greater than a triplet energy for an energy level of a triplet
state of the phosphorescent emitter in the further emission layer;
and the light-emitting organic region delivers an at least 5%
proportion of the light generated in the visible spectral range as
fluorescent light from singlet states of the fluorescent emitter in
the emission layer.
2. The light-emitting component according to claim 1, wherein the
light-emitting organic region delivers an at least 10% proportion
of the light generated in the visible spectral range as fluorescent
light of the singlet states of the fluorescent emitter in the
emission layer.
3. The light-emitting component according to claim 1, wherein light
generated in the light-emitting organic region delivers an at least
15% proportion of in the visible spectral range as fluorescent
light of the singlet states of the fluorescent emitter in the
emission layer.
4. The light-emitting component according to claim 1, wherein the
light-emitting organic region delivers an at least 20% proportion
of the light generated in the visible spectral range as fluorescent
light of the singlet states of the fluorescent emitter in the
emission layer.
5. The light-emitting component according to claim 1, wherein the
light-emitting organic region delivers an at least 25% proportion
of the light generated in the visible spectral range as fluorescent
light of the singlet states of the fluorescent emitter in the
emission layer.
6. The light-emitting component according to claim 1, wherein the
emission layer and the further emission layer adjoin one
another.
7. The light-emitting component according to claim 1, wherein the
emission layer and the further emission layer transport holes, and
in that a distance between a surface of the emission layer which
faces the electrode formed as a cathode and a surface of the
cathode which faces the emission layer is less than a distance
between a surface of the further emission layer which faces the
cathode and a surface of the cathode which faces the further
emission layer.
8. The light-emitting component according to claim 1, wherein the
emission layer and the further emission layer transport electrons,
and in that a distance between a surface of the emission layer
which faces the counter electrode formed as an anode and a surface
of the anode which faces the emission layer is less than a distance
between a surface of the further emission layer which faces the
anode and a surface of the anode which faces the further emission
layer.
9. The light-emitting component according to claim 1, wherein: the
light-emitting organic region comprises another emission layer
having one or a plurality of phosphorescent emitters emitting light
predominantly in the non-blue spectral range; the emission layer is
arranged between the further emission layer and the other emission
layer; the further emission layer transports electrons, a distance
between a surface of the further emission layer which faces the
cathode and a surface of the cathode which faces the further
emission layer being less than a distance between a surface of the
emission layer which faces the cathode and a surface of the cathode
which faces the emission layer, and also a distance between a
surface of the other emission layer which faces the cathode and a
surface of the cathode which faces the other emission layer; and
the other emission layer transports holes, a distance between a
surface of the other emission layer which faces the anode and a
surface of the anode which faces the other emission layer being
less than a distance between a surface of the emission layer which
faces the anode and a surface of the anode which faces the emission
layer, and also a distance between a surface of the further
emission layer which faces the anode and a surface of the anode
which faces the further emission layer.
10. The light-emitting component according to claim 1, wherein a
hole blocking layer is arranged between the emission layer and the
cathode, the hole blocking layer transporting electrons and an
organic material of the hole blocking layer having an HOMO level
which is at least about 0.3 eV lower than an HOMO level of the
fluorescent emitter in the emission layer.
11. The light-emitting component according claim 1, wherein an
electron blocking layer is arranged between the emission layer and
the anode, the electron blocking layer transporting holes and all
organic material of the electron blocking layer having an LUMO
level which is at least about 0.3 eV higher than an LUMO level of
the fluorescent emitter in the emission layer.
12. The light-emitting component according to claim 10, wherein a
minimum energy of singlet excitons and of triplet excitons in the
hole blocking layer or in the electron blocking layer is greater
than a minimum energy of singlet excitons and of triplet excitons
in the emission layer.
13. The light-emitting component according to claim 9, wherein
emission layer and/or the further emission layer and/or the other
emission layer are formed in multilayer fashion.
14. The light-emitting component according to claim 1, wherein the
emission layer has a thickness of between about 5 nm and about 50
nm.
15. The light-emitting component according to claim 1, wherein the
light-emitting organic region emits white light, and the or all of
the phosphorescent emitters in the further mission layer comprise
emitters emitting light in the red, orange, or yellow spectral
range.
16. The light-emitting component according to claim 1, wherein the
light-emitting organic region emits white light, and the or all of
the phosphorescent emitters in the further emission layer comprise
emitters emitting light in the red, orange or green spectral
range.
17. The light-emitting component according to claim 9, wherein the
light-emitting organic region emits white light, and the or all of
the phosphorescent emitters in the other emission layer comprise
emitters emitting light in the red, orange, or yellow spectral
range.
18. The light-emitting component according to claim 9, wherein the
light-emitting organic region emits white light, and the or all of
the phosphorescent emitters in the other emission layer comprise
emitters emitting light in the red, orange or green spectral
range.
19. The light-emitting component according to claim 1, wherein a
respective doped organic layer is arranged between the organic
light-emitting region and the electrode or between the organic
light-emitting region and the counterelectrode.
20. The light-emitting component according to claim 19, wherein the
respective doped organic layer comprises a layer which is p-doped
with an acceptor material or a layer which is n-doped with a donor
material.
21. The light-emitting component according to claim 1, wherein the
fluorescent emitter in the emission layer comprises an
organometallic compound or a complex compound with a metal having
an ordinal number of less than about 40.
22. The light-emitting component according to claim 1, wherein the
fluorescent emitter in the emission layer comprises an
electron-attracting substituent selected from the group consisting
of: a) halogens b) CN; c) halogenated or cyano-substituted alkanes
or alkenes; d) halogenated or cyano-substituted aryl radicals; and
e) boryl radicals.
23. The light-emitting component according to claim 1, wherein the
fluorescent emitter in the emission layer comprises an
electron-donating substituent selected from the group consisting
of: a) alkyl radicals; b) alkoxy radicals; c) aryl radicals with or
without substituents on the aryl; and d) amino groups.
24. The light-emitting component according to claim 1, wherein the
fluorescent emitter in the emission layer comprises a functional
group having an electron acceptor property selected from the group
consisting of: a) oxadiazole; b) triazole; c) benzotliadiazoles; d)
benzinidazoles and N-aryl-benzinidazoles; e) bipyridine; f)
cyanoviily; g) quinolines; h) quinoxalines; i) triarylboryl; j)
silol units; k) cyclooctatetraene; l) quinoid structures and
ketones, including quinoid thiophene derivatives; m) pyrazolines:
n) peiltaaryl cyclopentadiene; o) benzothiadiazoles; p)
oligo-para-phenyl with electron-attracting substituents; and q)
fluorenes and spiro-bifluorenes with electron-attracting
substituents.
25. The light emitting component according to claim 1, wherein the
fluorescent emitter in the emission layer comprises a functional
group having an electron acceptor property selected from the group
consisting of: a) triarylaillines; b) oligo-para-phenyl or
oligo-meta-phenyl; c) carbazoles; d) fluorene or spiro-bifluorenes;
e) phenylene-vinylene units; f) naphthalene: g) anthracene; h)
peryleile; i) pyrene; and j) thiophene.
Description
The invention relates to a light-emitting component, in particular
an organic light-emitting diode (OLED), having an electrode and a
counterelectrode and an organic region--arranged between the
electrode and the counterelectrode--with a light-emitting organic
region, which comprises an emission layer and a further emission
layer and which, upon application of an electrical voltage to the
electrode and the counterelectrode, is formed in a manner emitting
light in a plurality of colour ranges in the visible spectral
range, optionally through to white light.
PRIOR ART
Increased attention has been given in recent years to organic
light-emitting diodes which emit light in a plurality of colour
ranges through to white light. It is generally recognized that this
technology has a great potential for possible applications in the
field of illumination technology. White organic light-emitting
diodes are now reaching power efficiencies that lie in the region
of conventional electrical incandescent lamps (cf. Forrest et al.,
Adv. Mater. 7, 624 (2004)). Further improvements are expected.
Consequently, white OLEDs have the potential to form a significant
alternative to the illumination technologies that currently
dominate the market, for example incandescent lamps, halogen lamps,
low-voltage fluorescent tubes or the like.
In general, for OLEDs based on low molecular weight substances
("small molecules"), higher efficiencies can be obtained if
phosphorescent emitters are used. The latter can utilize up to 100%
of the excitons arising, which corresponds to an internal quantum
efficiency of 100%, in contrast to fluorescent emitters, which
utilize only the 25% proportion of singlet excitons (cf. Baldo et
al., Nature 395, 151 (1998); Baldo et al., Appl. Phys. Lett. 75, 4
(1999)). The term phosphorescent emitter denotes a material that is
able to emit phosphorescent light. In the same way, the term
fluorescent emitter denotes a material that is able to emit
fluorescent light.
According to the current state of the art, white OLEDs having the
highest known efficiencies are based on phosphorescent emitters
(cf. D'Andrade et al., Adv. Mater. 16, 624 (2004)).
However, a breakthrough of white OLEDs on the illumination market
still requires significant technical improvements. One challenge
that has not yet been satisfactorily solved at the present time is
to realize high-efficiency white OLEDs having a long lifetime.
White light sources corresponding to the state of the art have
lifetimes of approximately 750 h in the case of an incandescent
bulb and up to 10,000 h in the case of a fluorescent tube. In white
OLEDs, a contribution of blue or at least blue-green emitters is
generally required. The long-term stability of blue phosphorescent
emitters is extremely limited, however. Long lifetimes of blue OLED
emitters in the range of a few thousand to tens of thousands of
hours are only known for fluorescent emitters, which, however, can
only utilize a maximum of 25% of the excitons arising for the
generation of light. Green and red phosphorescent emitters in turn
are known with lifetimes in the range of several tens of thousands
to hundreds of thousands of hours.
In actual fact there has not hitherto been any satisfactory
solution to this problem. Lifetimes of while OLEDs which are based
on blue triplet emitters are too low for them to be suitable for
products capable of application. Therefore, all approaches for
introducing white OLEDs into production are currently directed
towards the use of fluorescent blue emitters. Although the
efficiencies are low in these approaches, the lifetime corresponds
to industry specifications in the region of a few 10,000 h at a few
100 cd/m.sup.2. Both phosphorescent and fluorescent emitters are
appropriate as red or green or orange emitters since the lifetimes
of the triplet emitters are already high enough for these colours.
There are numerous approaches for generating white light, for
example by means of mixing two colours, for example the colours
blue-green and orange, or mixing three colours, for example the
colours red, green and blue. The mixing in turn may be effected in
different ways, for example by means of: stacking the varicoloured
emitting OLEDs one above another, positioning varicoloured emitting
OLEDs one alongside another, introducing varicoloured emitters into
the same emission layer, or introducing the varicoloured emitters
into different emission layers, which may be separated from one
another with the aid of thin intermediate layers.
The document US 2003/0042848 A1 discloses an organic
electroluminescent display device having a substrate with a
plurality of sub-pixel regions for red, green and blue. The
sub-pixel regions each comprise a layer made of a light-emitting
material between two electrodes. The light-emitting material of at
least one of the plurality of sub-pixel regions comprises a
phosphorescent material.
THE INVENTION
The invention is based on the object of providing an improved
light-emitting component of the type mentioned in the introduction
which, in conjunction with high efficiency, also has a lifetime
that is long enough for illumination purposes.
This object is achieved according to the invention by means of a
light-emitting component according to independent Claim 1.
Dependent subclaims relate to advantageous refinements of the
invention.
The invention provides a light-emitting component, in particular
organic light-emitting diode, having an electrode and a
counterelectrode and an organic region--arranged between the
electrode and the counterelectrode--with a light-emitting organic
region, which comprises an emission layer and a further emission
layer and which, upon application of an electrical voltage to the
electrode and the counterelectrode, is formed in a manner emitting
light in a plurality of colour ranges in the visible spectral
range, optionally through to white light, in which case: the
emission layer comprises a fluorescent emitter which emits light
predominantly in the blue or in the blue-green spectral range; the
further emission layer comprises one or a plurality of
phosphorescent emitters emitting light predominantly in the
non-blue spectral range; a triplet energy for an energy level of a
triplet state of the fluorescent emitter in the emission layer is
greater than a triplet energy for an energy level of a triplet
state of the phosphorescent emitter in the further emission layer;
and an at least 5% proportion of the light generated in the
light-emitting organic region is formed in the visible spectral
range as fluorescent light from singlet states of the fluorescent
emitter in the emission layer.
The invention has the advantage over the prior art of providing a
high-efficiency and at the same time long-lived light-emitting
component for light in a plurality of colour ranges in the visible
spectral range through to white light. The problem that blue
fluorescent emitters admittedly have a long lifetime but are not
very efficient is overcome by virtue of the fact that triplet
excitons which arise upon application of the electrical energy in
the emitter which emits light predominantly in the blue or in the
blue-green spectral range and which normally decompose
non-radiatively are utilized for light emission in particular on
account of the configuration of the triplet energy for the energy
level of the triplet state of the fluorescent emitter in the
emission layer and of the triplet energy for the energy level of
the triplet state of the phosphorescent emitter in the further
emission layer. In this way, it is possible to transfer the energy
from the triplet state of the fluorescent emitter in the emission
layer to the triplet state of the phosphorescent emitter in the
further emission layer. At the same time, light is emitted in the
blue or blue-green spectral range upon the decomposition of the
singlet states of the fluorescent emitter in the emission layer. In
the organic light-emitting region of the component, the following
processes, in particular, proceed: a) the predominant majority of
the excitons formed in the organic light-emitting region when the
electrical energy is supplied in the component is formed on the
fluorescent emitter in the emission layer in the form of singlet
excitons and triplet excitons; b) a portion of the singlet excitons
decomposes radiatively with emission of blue or blue-green
fluorescent light; c) the triplet excitons diffuse by means of
excitation energy transfer from the emission layer into the further
emission layer containing the phosphorescent emitter; and d) the
triplet excitons of the phosphorescent emitter decompose
radiatively with emission of non-blue phosphorescent light. In this
way, the energy fed to the organic region arranged between the
electrode and the counter electrode is converted with high
efficiency for the generation of light having different wavelengths
in the visible spectral range.
Therefore, one essential aspect is the combination of long-lived
emitter materials with high-efficiency emitter materials. As
already mentioned, a white-emitting OLED is always based on the
fact that one or a plurality of emitter(s) emitting blue or
blue-green is/are implemented. A fluorescent emitter material is
used here as the emitter since the lifetimes of phosphorescent blue
or blue-green emitters are orders of magnitude too low for
practically all conceivable applications. With fluorescent blue
emitters, by contrast, lifetimes of more than 20,000 h with a
brightness of 1000 cd/m.sup.2 are already achieved today. The
situation is different for red and green emitters. Here long
lifetimes are actually already obtained even with phosphorescent
emitters, for example 30,000 h with a brightness of 500 cd/m.sup.2
for red OLEDs. The invention makes it possible, despite the
utilization of a fluorescent blue emitter, also to utilize the
majority, for example approximately 80 to 90%, of the triplet
excitons produced in the blue-emitting emission layer for the
generation of light and thus to increase the current efficiency of
the component by a factor of 1.5 to 2.
One expedient refinement of the invention provides for an at least
10% proportion of the light generated in the light-emitting organic
region to be formed in the visible spectral range as fluorescent
light of the singlet states of the fluorescent emitter in the
emission layer.
In one refinement of the invention, it may be provided that an at
least 15% proportion of the light generated in the light-emitting
organic region is formed in the visible spectral range as
fluorescent light of the singlet states of the fluorescent emitter
in the emission layer.
One preferred development of the invention provides for an at least
20% proportion of the light generated in the light-emitting organic
region to be formed in the visible spectral range as fluorescent
light of the singlet states of the fluorescent emitter in the
emission layer.
Preferably, one development of the invention may provide for an at
least 25% proportion of the light generated in the light-emitting
organic region to be formed in the visible spectral range as
fluorescent light of the singlet states of the fluorescent emitter
in the emission layer.
In one expedient development of the invention, it is provided that
a hole blocking layer is arranged between the emission layer and
the cathode, the hole blocking layer transporting electrons and an
organic material of the hole blocking layer having an HOMO level
which is at least approximately 0.3 eV lower than an HOMO level of
the fluorescent emitter in the emission layer.
One expedient development of the invention may provide for an
electron blocking layer to be arranged between the emission layer
and the anode, the electron blocking layer transporting holes and
an organic material of the electron blocking layer having an LUMO
level which is at least approximately 0.3 eV higher than an LUMO
level of the fluorescent emitter in the emission layer.
One embodiment of the invention preferably provides for the
emission layer and/or the further emission layer and/or the other
emission layer to be formed in multilayer fashion.
One advantageous development of the invention provides for the
emission layer to have a thickness of between approximately 5 nm
and approximately 50 nm.
One expedient development of the invention may provide for the
light-emitting organic region to be formed in a manner emitting
white light, the or all of the phosphorescent emitters in the
further emission layer being emitters emitting light in the red,
orange or yellow spectral range.
In one preferred embodiment of the invention, it is provided that
the light-emitting organic region is formed in a manner emitting
white light, the or all of the phosphorescent emitters in the
further emission layer being emitters emitting light in the red,
orange or green spectral range.
One advantageous refinement of the invention provides for the
light-emitting organic region to be formed in a manner emitting
white light, the or all of the phosphorescent emitters in the other
emission layer being emitters emitting light in the red, orange or
yellow spectral range.
In one development of the invention, it is expediently provided
that the light-emitting organic region is formed in a manner
emitting white light, the or all of the phosphorescent emitters in
the other emission layer being emitters emitting light in the red,
orange or green spectral range.
One expedient development of the invention provides for the
fluorescent emitter in the emission layer to be an organometallic
compound or a complex compound with a metal having an ordinal
number of less than 40.
One preferred development of the invention may provide for the
fluorescent emitter in the emission layer to comprise an
electron-attracting substituent from one of the following classes:
a) halogens such as fluorine, chlorine, iodine or bromine; b) CN;
c) halogenated or cyano-substituted alkanes or alkenes, in
particular trifluoromethyl, pentafluoroethyl, cyanovinyl,
dicyanovinyl, tricyanovinyl; d) halogenated or cyano-substituted
aryl radicals, in particular pentafluorophenyl; or e) boryl
radicals, in particular dialkylboryl, dialkylboryl having
substituents on the alkyl groups, diarylboryl or diarylboryl having
substituents on the aryl groups.
One advantageous development of the invention provides for the
fluorescent emitter in the emission layer to comprise an
electron-donating substituent of one of the following classes: a)
alkyl radicals such as methyl, ethyl, tert-butyl, isopropyl; b)
alkoxy radicals; c) aryl radicals with or without substituents on
the aryl, in particular tolyl and mesityl; or d) amino groups, in
particular NH2, dialkylamine, diarylamine and diarylamine having
substituents on the aryl.
One embodiment of the invention expediently provides for the
fluorescent emitter in the emission layer (EML1) to comprise a
functional group having an electron acceptor property from one of
the following classes:
a) oxadiazole
b) triazole
c) benzothiadiazoles
d) benzimidazoles and N-aryl-benzimidazoles
e) bipyridine
f) cyanovinyl
g) quinolines
h) quinoxalines
i) triarylboryl
j) silol units, in particular derivative groups of
silacyclopentadiene
k) cyclooctatetraene
l) quinoid structures and ketones, including quinoid thiophene
derivatives
m) pyrazolines
n) pentaaryl cyclopentadiene
o) benzothiadiazoles
p) oligo-para-phenyl with electron-attracting substituents, or
q) fluorenes and spiro-bifluorenes with electron-attracting
substituents.
In one expedient development of the invention, it is provided that
the fluorescent emitter in the emission layer (EML1) comprises a
functional group having an electron acceptor property from one of
the following classes:
a) triarylamines
b) oligo-para-phenyl or oligo-meta-phenyl
c) carbazoles
d) fluorene or spiro-bifluorenes
e) phenylene-vinylene units
f) naphthalene
g) anthracene
h) perylene
i) pyrene or
j) thiophene.
EXEMPLARY EMBODIMENTS OF THE INVENTION
The invention is explained in more detail below on the basis of
exemplary embodiments with reference to figures of the drawing, in
which:
FIG. 1 shows a schematic illustration of a layer arrangement of an
organic light-emitting diode;
FIG. 2 shows a schematic illustration of a layer arrangement of a
further organic light-emitting diode;
FIG. 3 shows a schematic illustration of a layer arrangement of an
organic light-emitting diode in which a main recombination zone is
formed in a targeted manner according to a first embodiment;
FIG. 4 shows a schematic illustration of a layer arrangement of an
organic light-emitting diode in which the main recombination zone
is formed in a targeted manner according to a second
embodiment;
FIG. 5 shows a schematic illustration of a layer arrangement of an
organic light-emitting diode in which the main recombination zone
is formed in a targeted manner according to a third embodiment;
FIG. 6 shows a CIE chromaticity diagram;
FIG. 7 shows a model spectrum for an emitter material emitting
light in the blue spectral range;
FIG. 8 shows a diagram for comparison of the necessary blue
emission light when generating white light for different light
emissions in the green and red spectral ranges;
FIG. 9 shows an electroluminescence spectrum for an organic
reference light-emitting diode;
FIG. 10 shows an electroluminescence spectrum for a first organic
light-emitting diode according to the invention;
FIG. 11 shows an electroluminescence spectrum for a second organic
light-emitting diode according to the invention; and
FIG. 12 shows structural formulae for molecules of an emitter
material.
FIG. 1 shows a schematic illustration of a layer arrangement of an
organic light-emitting diode (OLED). Two electrodes, namely an
anode 1 and a cathode 2, are formed, between which is arranged an
organic region with a hole transport layer 3 made of an organic
substance doped with an acceptor material, an organic
light-emitting region 4 with an emission layer EML1 and a further
emission layer EML2 and also an electron transport layer 5 made of
an organic substance doped with a donor material. The emission
layer EML1 comprises a fluorescent emitter which emits light
predominantly in the blue or in the blue-green spectral range. In
an alternative embodiment, the emission layer EML1 comprises a
plurality of fluorescent emitters emitting light predominantly in
the blue or in the blue-green spectral range. The further emission
layer EML2 comprises one or a plurality of phosphorescent
emitter(s) emitting light predominantly in the non-blue spectral
range.
FIG. 2 shows a schematic illustration of a layer arrangement of a
further OLED. For identical features, the same reference symbols as
in FIG. 1 are used in FIG. 2. In the case of the OLED in FIG. 2, a
hole blocking layer 6 and an electron blocking layer 7 are
additionally provided in the organic region.
Within the OLED, the following processes proceed during operation:
charge carriers in the form of holes are injected through the anode
1 into the hole transport layer 3, migrate through the hole
transport layer 3 and reach, if appropriate through the electron
blocking layer 7, the light-emitting organic region 4; charge
carriers in the form of electrons are injected through the cathode
2 into the electron transport layer 5, migrate through the electron
transport layer 5 and reach, if appropriate through the hole
blocking layer 6, the light-emitting organic region 4; and in the
organic light-emitting region 4, holes and electrons meet one
another and recombine to form excited states, so-called excitons,
which are formed in triplet and singlet states.
With regard to their transport or conduction properties, the
organic materials used may conduct electrons/transport electrons,
in which case said materials may then also conduct holes/transport
holes. It holds true in the same way that an organic material
designated as one which conducts holes/transports holes may also
conduct electrons/transport electrons, so that the two properties
are not mutually exclusive.
It is advantageous in the implementation of the arrangement if the
energetic distance between the singlet state and the triplet state
of the emitter which emits in the blue or blue-green spectral range
in the emission layer EML1 is as small as possible, thereby
enabling an exothermic energy transfer to the triplet states of the
emitters which emit in the non-blue spectral range in the further
emission layer EML2. The following design principles are preferably
utilized for the emitter emitting in the blue or blue-green
spectral range: i) the emitter material is an organometallic
compound or a complex compound, the lowest excitation state
exhibiting "metal-to-ligand-charge-transfer" character (MLCT),
which means that, in the event of excitation, an electron is lifted
from an orbital which is predominantly localized on the metal to an
orbital which is predominantly localized on the organic ligand. ii)
the emitter material is push-pull-substituted, which means that at
least one substituent having electron-donating character and at
least another substituent having electron-attracting character are
attached to a basic skeleton via which the pi-electrons of the HOMO
and LUMO wave functions (HOMO--"Highest Occupied Molecular
Orbital"; LUMO--"Lowest Unoccupied Molecular Orbital") are
delocalized, so that the centroid of the HOMO wave function is
shifted towards the electron-donating substituents and the centroid
of the LUMO wave function is shifted towards the
electron-attracting substituents and the spatial overlap of HOMO
and LUMO wave functions thus decreases in comparison with a
corresponding molecule without substituents or one with exclusively
electron-attracting or exclusively electron-donating substituents.
iii) The pi-electron system of the emitter material extends over
functional groups which in part rather exhibit electron acceptor
character and on which the LUMO wave function is correspondingly
concentrated, and other functional groups which rather exhibit
electron donor character and on which the HOMO wave function is
consequently concentrated.
FIG. 12 shows structural formulae for molecules of emitters
emitting in the blue or blue-green spectral range, in which the
abovementioned design rules are taken into account.
The OLEDs according to FIGS. 1 and 2 are distinguished by the fact
that the charge carrier recombination is predominantly effected in
the emission layer EML1, as a result of which both triplet and
singlet excitons form in the emission layer EML1. The position of a
main recombination zone is influenced in a targeted manner. The
term main recombination zone denotes a spatial zone within the
region of the organic layers between the anode 1 and the cathode 2
in which at least approximately 50% of the injected charge carriers
recombine.
Embodiments for influencing the main recombination zone in a
targeted manner are described below with reference to FIGS. 3 to 5.
The same reference symbols as in FIGS. 1 and 2 are used for
identical features in FIGS. 3 to 5.
In an OLED according to FIG. 3, the position of the main
recombination zone is influenced by virtue of the fact that the
further emission layer EML2 preferably transports holes as charge
carriers, that is to say that the further emission layer EML2 is
formed such that it conducts/transports holes, and is arranged
directly adjacent to the electron blocking layer 7 or, if such a
layer is not present in another embodiment (not illustrated),
directly adjacent to the hole transport layer 3.
In this case, the emission layer EML1 with the fluorescent emitter
is in direct proximity to the hole blocking layer 6 or, if this
layer is not present, in direct proximity to the electron transport
layer 5. In this embodiment, it is furthermore provided that the
emission layer EML1 also preferably transports holes, that is to
say that the emission layer EML1 is formed such that it
conducts/transports holes. It is ensured in this way that the holes
meet the electrons, and recombine with them to form excitons,
preferably in the vicinity of the interface between the emission
layer EML1 and the hole blocking layer 6 or, if this layer is not
provided, the electron transport layer 5.
Although the main recombination zone is situated in the vicinity of
said interface, the recombination takes place almost exclusively in
the emission layer EML1 and not in the adjoining layer. An
arrangement such as this prevents, in particular, electrons from
being transported deep into the emission layers EML1, EML2 which
would result in a widening of the emission zone or, in the worst
case, might lead to a recombination of the electrons and the holes
in the further emission layer EML2, which would counteract the
intended optimized generation of white light.
A particularly advantageous refinement of the embodiment with the
hole blocking layer 6 provides for a minimum energy of singlet
excitons and of triplet excitons in the hole blocking layer 6 to be
greater than a minimum energy of singlet excitons and of triplet
excitons in the emission layer EML1. What is thereby achieved is
that the triplet excitons which arise in the emission layer EML1
can diffuse only in the direction of the further emission layer
EML2, whereby the efficiency of the OLED increases. If no hole
blocking layer is provided, this requirement made of the exciton
energy, for an advantageous refinement of the invention, applies
analogously to the electron transport layer.
An alternative arrangement for controlling the position of the main
recombination zone is realized in an embodiment of an OLED
according to FIG. 4 by virtue of the fact that the further emission
layer EML2 with the phosphorescent emitter preferably transports
electrons as charge carriers, that is to say that the further
emission layer EML2 is formed such that it conducts/transports
electrons, and is arranged directly adjacent to the hole blocking
layer 6 or, if such a layer is not present in another embodiment
(not illustrated), directly adjacent to the electron transport
layer 5.
In this case, the emission layer EML1 with the fluorescent emitter
is in direct proximity to the electron blocking layer 7 or, if this
layer is not present, in direct proximity to the hole transport
layer 3. In this embodiment, it is furthermore provided that the
emission layer EML1 preferably transports electrons as charge
carriers, that is to say that the emission layer EML1 is formed
such that it conducts/transports electrons. This ensures that the
electrons meet the holes, and recombine with them to form excitons,
preferably in the vicinity of the interface between the emission
layer EML1 and the electron blocking layer 7 or, if this layer is
not provided, the hole transport layer 3. Although the main
recombination zone is situated in the vicinity of said interface,
the recombination takes place almost exclusively in the emission
layer EML1 and not in the adjoining layer. An arrangement such as
this prevents, in particular, holes from being transported deep
into the emission layers EML1, EML2 which would result in a
widening of the emission zone or, in the worst case, might lead to
a recombination of the electrons and the holes in the further
emission layer EML2, which would counteract the intended optimized
generation of white light.
A particularly advantageous refinement of the embodiment with the
electron blocking layer 7 provides for a minimum energy of singlet
excitons and of triplet excitons in the electron blocking layer 7
to be greater than a minimum energy of singlet excitons and of
triplet excitons in the emission layer EML1. What is thereby
achieved is that the triplet excitons which arise in the emission
layer EML1 can diffuse only in the direction of the further
emission layer EML2, whereby the efficiency of the OLED increases.
If no electron blocking layer is provided, this requirement made of
the exciton energy, for an advantageous refinement of the
invention, applies analogously to the hole transport layer.
For the exemplary embodiments explained, the thickness of the
emission layer EML1 with the fluorescent emitter(s) is to be chosen
such that it is greater than a diffusion length of the singlet
excitons, which is in the region of approximately 20 nm for example
for Alq.sub.3, but less than a diffusion length of the triplet
excitons, which is in the region of approximately 140 nm for
example for Alq.sub.3 (cf. A. Hunze, Organische Leuchtdioden auf
Basis von dotierten Emissionsschichten [Organic light-emitting
diodes based on doped emission layers], Shaker Verlag, 2003). These
diffusion lengths are different, of course, for other materials. It
can generally be assumed that the diffusion length of the singlet
excitons is between approximately 5 nm and approximately 30 nm.
Therefore, one advantageous embodiment provides for a thickness of
the emission layer EML1 of between approximately 7 nm and
approximately 35 nm depending on the fluorescent emitter layer
used.
On the other hand, this layer ought not to become too thick since
resistive losses then become greater and, moreover, increasingly
fewer triplet excitons reach the further emission layer EML2 with
the phosphorescent emitter(s). Therefore, the emission layer EML1
with the fluorescent emitter(s) is preferably thinner than
approximately 100 nm, more preferably thinner than approximately 60
nm.
FIG. 5 shows an OLED in which a further possibility for influencing
the position of the main recombination zone is formed by virtue of
the fact that the emission layer EML1 is arranged between the
further emission layer EML2 with the phosphorescent emitter(s) and
another emission layer EML3 having one or a plurality of
phosphorescent emitter(s), the emission layer EML2 preferably
transporting electrons and the other emission layer EML3 preferably
transporting holes. If appropriate, the two emission layers EML2,
EML3 can emit light in different wavelength ranges. In this case,
it is necessary for the emission layer EML1 to have an ambipolar
transport behaviour, that is to say for the emission layer EML1 to
have both electron transport and hole transport properties, so that
the main recombination zone does not form at the interface between
one of the two emission layers EML2, EML3 and the emission layer
EML1 but rather in the emission layer EML1. In this embodiment, the
thickness of the emission layer EML1 is to be chosen such that the
thickness is greater than approximately twice the diffusion length
of the singlet excitons but less than approximately twice the
diffusion length of the triplet excitons. An advantageous
embodiment provides for the thickness of the emission layer EML1 to
be approximately 15 nm to approximately 100 nm.
Calculations for optimizing the efficiency with regard to the
colour spectrum of the embodiments described are explained
below.
In order to obtain a white light emission by a plurality of
coloured emitters, it is necessary for the proportion of the
different colour components to be well balanced, which means that
the proportions of the individual components with respect to one
another must be in a specific, suitable relationship. A description
of the quality of white light can be given by means of the CIE
colour coordinates x and y described in the CIE chromaticity
diagram of 1931. In this case, an optimum white hue is given by the
colour coordinates x=0.33 and y=0.33. A while OLED is understood
here to be an OLED which emits light whose colour coordinate lies
within the white region of the CIE chromaticity diagram as
illustrated in FIG. 6. The "Colour Rendering Index" (CRI) gives an
indication about how many components of the natural solar spectrum
are contained in a white light source. CRI values of above 65 mean
that the white is already a very "saturated" white.
In order to obtain high efficiencies for the optimum colour locus
of 0.33/0.33, it is desirable to use the individual emitter
materials such that the proportion of the blue/blue-green light is
as far as possible 25% of the total light emitted by the OLED,
since the theoretically obtainable internal quantum efficiency is
maximal in this case. On the basis of these considerations,
calculations were carried out with respect to the quantum
efficiencies that can be obtained by means of the invention.
The calculations use different emitter spectra for red, green and
blue emission as a basis. Electroluminescence spectra of known
phosphorescent emitters which correspond to the state of the art
("SOA") were used for green and red, whereas besides an
electroluminescence spectrum of a fluorescent emitter corresponding
to the state of the art, an artificially generated model
spectrum--which is shown in FIG. 7--was also used for the
fluorescent emitter emitting in the blue/blue-green spectral
range.
These spectra and the photometric radiation equivalents that can be
derived directly from them were used as a basis to calculate what
proportion of light generation by the individual emitters is
required to obtain a balanced white light emission at a reference
point having the CIE colour coordinates 0.33/0.33. The calculations
were carried out by means of a software for the simulation of RGB
display elements. This software makes it possible to calculate the
light proportion which is required by the individual pixels in
order to obtain a colour mixing for a specific point within the CIE
diagram of 1931. The calculations were carried out for white light
generation by means of a blue, green and red emitter. These
considerations can be applied to the use of an emitter system
having two emitters.
The software used permits the calculation of the current which is
required by the individual pixels of a colour if the colour
coordinates of the pure emitter and also its current efficiency are
known. The following assumptions were made for using the program
for the calculations. The quantum efficiencies of all the emitters
are regarded as identical if an identical emission mechanism is
taken as a basis, for example for two phosphorescent triplet
emitters. It was furthermore assumed that the quantum efficiency of
a fluorescent singlet emitter is 25% of the quantum efficiency of a
triplet emitter.
In order to feed the values for the current efficiency of the
emitters into the software used, values proportional to the
photometric radiation equivalents of the individual emitters were
used. In the case of singlet emitters, values proportional to 25%
of the photometric radiation equivalent were used in order to take
account of the reduced quantum efficiency in comparison with
triplet emitters.
In the case of the invention, however, the situation deviates from
the assumptions outlined previously since, in this case, for the
singlet emitter emitting in the blue spectral range, 100% of the
corresponding photometric radiation equivalents was used for the
calculation. This assumption can be made since, in the case set
forth, all the excitons are formed in the emission layer EML1 on
the fluorescent emitter(s) and the singlet excitons are emitted
there as light. The triplet excitons, by contrast, diffuse into the
emission layers EML2/EML3 with the triplet emitter(s) where they
decompose radiantly. On account of this circumstance, a white
emission system in an organic light-emitting diode which utilizes
this mechanism can achieve a maximum theoretical quantum efficiency
of 100% if 25% of the light emission which is required for
generating the white light is emitted by the fluorescent blue
emitter and 75% by the triplet emitters.
A first aim of the calculations was to show how a well-balanced and
high-efficiency white light emission can be realized on the basis
of the invention if a realistic blue emission spectrum is taken as
a basis. These calculations were carried out with a blue model
spectrum and the electroluminescence spectrum of a blue fluorescent
emitter corresponding to the state of the art (cf. FIG. 8).
In the case of white light generation on the basis of the blue
model spectrum, this requires 28% of the emission by the
fluorescent blue emitter, whereas 37% of the total light emission
would have to be generated by the fluorescent blue emitter when
using the known blue electroluminescence spectrum. It emerges from
this that the theoretical efficiency for the model considered,
compared with an efficiency of 100% in the case of a perfect white
light emission exclusively on the basis of triplet emission, is 89%
in the case of the model spectrum, whereas this value decreases to
68% in the case of the known blue electroluminescence spectrum.
These values can now be compared with other approaches for white
light OLEDs, based on exclusively fluorescent singlet emitters or
on a blue singlet emitter and a red and green triplet emitter; this
comparison is reproduced in table 1.
TABLE-US-00001 TABLE 1 Total photometric radiation equivalent Total
B[%] G[%] R[%] relative efficiency Exclusively Blue model 0.28 0.33
0.39 55.32725 25% singlet spectrum emitters Blue SOA 0.37 0.24 0.39
56.0895 25% EL spectrum Blue singlet, Blue model 0.59 0.2 0.21
129.77375 55.80% red and green triplet Blue SOA 0.71 0.11 0.18
104.37775 46.70% EL spectrum Exemplary Blue model 0.28 0.33 0.39
199.1781 89% embodiment of the invention Blue SOA 0.37 0.24 0.39
179.4864 68% EL spectrum Exclusively Blue model 0.28 0.33 0.39
221.309 100% triplet emitters Blue SOA 0.37 0.24 0.39 224.358 100%
EL spectrum
The values from table 1 verify that the theoretical limit of white
light generation on the basis of the invention is at least 90%. In
the case of an even more favourable blue spectrum, it may, if
appropriate, extend even closer to the upper limit of 100% which
can be achieved in the case of pure light generation on triplet
emitters.
Consequently, the invention makes it possible to increase the
internal quantum efficiency for white light generation by means of
electroluminescence when using a fluorescent blue emitter to above
65%, and in an improved embodiment to above 90%. This means a
drastic increase in exciton utilization compared with conventional
OLEDs on the basis of a fluorescent blue emitter and phosphorescent
green and red emitters and also compared with purely fluorescent
white OLEDs.
Exemplary embodiments of the OLEDs are described below in order to
elucidate the invention further.
Firstly, a reference OLED having the following layer arrangement
was produced: 1) anode: indium tin oxide (ITO) 2) p-doped hole
transport layer: 80 nm,
4,4',4''-tris(N,N-diphenylamino)triphenylamine (Starburst TDATA)
doped with tetrafluoro-tetracyano-quinodimethane (F4-TCNQ) 3)
hole-side intermediate layer: 10 nm
N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine (TPD) 4) blue
emission layer: 20 nm .alpha.-NPD 5) electron-side intermediate
layer: 10 nm bathophenanthroline (Bphen) 6) n-doped electron
transport layer: 30 nm Bphen doped with Cs 7) cathode: 100 nm
aluminium
This pin-OLED exhibits, in accordance with FIG. 9, a blue emission
with a maximum at 440 nm. The sample has a current efficiency of
1.8 cd/A and a quantum efficiency of 1.5% at a brightness of 10,000
cd/m.sup.2.
A first exemplary embodiment of an OLED according to the invention
provides the following layer structure: 1) anode: indium tin oxide
(ITO) 2) p-doped hole transport layer: 80 nm,
4,4',4''-tris(N,N-diphenylamino)triphenylamine (Starburst TDATA)
doped with tetrafluoro-tetracyano-quinodimethane (F4-TCNQ) 3)
hole-side intermediate layer: 10 nm
N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine (TPD) 4)
orange-red emission layer: 10 nm
bis[N-(1-naphthyl)-N-phenyl]benzidine (.alpha.-NPD) doped with
iridium (III) bis(2-methyldibenzo[f,h]quinoxaline)
(acetylacetonate))(RE076,ADS) 5) blue emission layer: 20 nm
.alpha.-NPD 6) electron-side intermediate layer: 10 nm
bathophenanthroline (Bphen) 7) n-doped electron transport layer: 30
nm Bphen doped with Cs 8) cathode: 100 nm aluminium
This OLED is a white pin-OLED which exhibits a brightness of above
10,000 cd/m.sup.2 at a voltage of 6V. The electroluminescence
spectrum has two peaks at 450 nm and 610 nm in accordance with FIG.
10. The CIE colour coordinates are x=0.33 and y=0.22. The sample
has a current efficiency of 5.7 cd/A and a quantum efficiency of
4.2% at a brightness of 10,000 cd/m.sup.2. Upon consideration of
the electroluminescence spectrum of this OLED, both the emission of
.alpha.-NPD and that of the orange-red emitter RE076 are clearly
evident. FIG. 9 shows the spectrum of a pure emission of the
.alpha.-NPD in comparison with this.
On account of the energetic position at the contract area of Bphen
(HOMO .about.-6.4 eV, LUMO .about.-3.0 eV, He et al., Appl. Phys.
Lett. 85(17), 3911 (2004)) and .alpha.-NPD (HOMO .about.-5.7 eV,
LUMO .about.-2.6 eV, Baldo et al., Appl. Phys. Lett. 75(1), 4
(1999)), in this structure the recombination of holes and electrons
is necessarily effected at this contact area. Therefore, initially
only a blue singlet emission of the .alpha.-NPD is expected. In
actual fact, however, a considerable red emission is furthermore
also manifested as well. Furthermore, the sample exhibits a
significantly higher current and quantum efficiency than the
comparison sample without the emission layer with the
phosphorescent emitter, which can be explained to this extent only
by an efficient utilization of the triplet excitons formed at the
Bphen/.alpha.-NPD interface by the red triplet emitter.
A second exemplary embodiment of an OLED according to the invention
provides the following layer structure: 1) anode: indium tin oxide
(ITO) 2) p-doped hole transport layer: 60 nm,
4,4',4''-tris(N,N-diphenylamino)triphenylamine (Starburst TDATA)
doped with tetrafluoro-tetracyano-quinodimethane (F4-TCNQ) 3)
hole-side intermediate layer: 10 nm
2,2',7,7'tetrakis(N,N-diphenylamino)-9,9'-spirobifluorene
(spiro-TAD) 4) orange-red emission layer: 10 nm
bis[N-(1-naphthyl)-N-phenyl]benzidine (.alpha.-NPD) doped with
iridium (III) bis(2-methyldibenzo[f,h]quinoxaline)
(acetylacetonate)(RE076, ADS) 5) blue emission layer: 30 nm
.alpha.-NPD 6) green emission layer:
1,3,5-Tri(phenyl-2-benzimidazole)benzene (TPBI) doped with
fac-tris(2-phenylpyridine)iridium (Ir(ppy).sub.3) 7) electron-side
intermediate layer: 10 nm bathophenanthroline (Bphen) 8) n-doped
electron transport layer: 30 nm Bphen doped with Cs 9) cathode: 100
nm aluminium
This OLED is a white pin-OLED which, in accordance with FIG. 11,
exhibits an emission with colour coordinates of 0.42/0.44. The
sample has a current efficiency of 11 cd/A at an operating voltage
of 4.3V and a brightness of 2000 cd/m.sup.2. In this structure,
too, electrons and holes recombine predominantly in the .alpha.-NPD
layer, in part also in the emission layer with the green emitter,
since .alpha.-NPD is a hole transporter (cf. Kido et al., App.
Phys. Lett. 73 (20), 2866 (1998)). While singlet excitons which are
formed in the .alpha.-NPD decompose to radiate blue, the triplet
excitons generated there diffuse into the adjacent emission layers
(orange-red and green), where they decompose in part radiatively
with emission of light having a longer wavelength.
The features of the invention which are disclosed in the above
description, the claims and the drawings may be of importance both
individually and in any desired combination for the realization of
the invention in its various embodiments.
* * * * *
References